How to Remove Conformal Coatings

Despite the protection of conformal coatings, failure of PCBs and other electronic components can occur for several reasons, including:

• Incorrect selection of coating methods/materials related to the electronics’ end use
• Poor manufacture or stresses to the component during operation
• Challenges to the coating’s integrity – bubbles/voids, inadequate masking, poor adhesion/surface finish, uneven thickness

To determine the right removal method, these key factors must be identified:

• The type of coating material used and application method
• Age and thickness of the coating
• The PCB’s components and their position on the board
• The coating’s effect on the substrate

Removal Methods

Once these have been identified, determination of the appropriate removal method is possible. The major removal methods include:

• Chemical solvents are most frequently used for removal for liquid coatings. However, because no single solvent will remove every coating material with the same degree of efficiency, selection of best chemical solvent is dependent on coating-type. The solvent butyrolactone is frequently used for acrylic removal. Methylene chloride or hydrocarbon-based solvents are recommended for silicone. Methanol-base/alkaline activators and ethylene glycol ether-base/alkaline activators are used for urethane. Solvents are generally ineffective for epoxy and chemically-inert Parylene.
• Cost-effective laser ablation offers precise removal, to a single micron, but because each laser pulse separates only a minute segment of the existing film coverage, it can be slow. Generally a one-step procedure, laser is particularly good for Parylene removal, re-converting the solid film back into a gas or plasma.
• Low-cost, environmentally-safe micro-abrasive blasting (abrasion) can be focused onto minute board segments (e.g., an individual test node) or an entire PCB. Automated or manual methods of micro abrasion direct project-specific formulas of abrasive media, inert-gas or dry air through a tiny nozzle onto targeted surfaces. Resultant coating debris are disposed by filtration and grounding devices dispel electrostatic potential. Abrasion can effectively remove all liquid coatings and Parylene.
• Mechanical removal techniques include grinding, scraping, cutting or sanding coating from the surface. Thorough masking of non-removal surfaces is time consuming but necessary. Less dependable than other removal techniques, poor mechanical processing can damage the coating and PCB. Acrylic, epoxy and urethane coatings respond to mechanical methods, as do thicker silicone films. If needed, scraping can also work for Parylene.
• Plasma is frequently used when highly-selective coating removal from specific components within an assembly is required; it is also effective for removing coatings from entire circuit boards. Parylene coatings, in particular, respond well to plasma spot-removal.
• Peeling is suggested only for specialized removal. For instance, thickly-applied silicone films can respond well when a dull knife or blade is used to slit the film and then an operator peels it from the PCB by hand.
• Thermal removal methods such as a soldering iron can generate difficult-to-manage toxic fumes at high temperatures. Long exposures to high heat can also overheat temperature-sensitive components, negatively impacting solder joints, leaving surface residue and delaminating/ discoloring the remaining surface. Frequently used for durable epoxy coatings, thermal methods have some use for acrylic, silicone and urethane; spot-removal is most recommended, especially for Parylene.

Industry Standards

Industry standards provide appropriate process guidelines for conformal coating removal. IPC-7711/7721, imparts best-practice procedures for removing conformal films from PCBs. The standards detail coating removal methods determined by their impact on the film, its thickness and effect on the substrate. IPC-CC-830 defines conformance guidelines for all conformal coatings; film removal is frequently required when IPC-CC-830 requirements for each coating type’s overall quality conformance are unmet during application.

Skill and Conformance Requirements

Operator experience and skill influence coating removal. Appropriate material identification is necessary prior to beginning removal. Better equipped to correctly identify coating characteristics, skilled professionals determine the best removal procedures with greater accuracy; they can also implement removal procedures within the constraints of conformance issues like the PCB electrical component’s environmental, functional and serviceability requirements, improving the removal process.